[solved]sorting array of path pointers by distance in 3D model
lazarus / 62-General

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tygraphics · 2024-08-09 16:19

Hi,

I am working on a 3D vector application project using Free Pascal, and
I need to sort an array of pointers referencing Path records. Each
Path record contains a field that stores the distance from a camera to
the center point of that path. The goal is to sort this array of
pointers in descending order based on the distance values so that
during the rendering process, paths that are furthest away from the
camera are drawn first, and those closest are drawn last.

I appreciate any help you can provide.
Thomas Young

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Hartmut · 2024-08-09 16:55

can you show as the declaration of the records and the array of
pointers? And how many data (about) do you want to sort?

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tooldeveloper · 2024-08-09 18:26

The TFPObjectList class in the Contnrs unit may solve your issue. 
Assuming I am reading your question correctly then below is a quick
example of doing what I think you are asking.

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i uses 
Contnrs; { TForm1 }   type    T3DVector = record      CamDistance :
Int64;    end;    P3DVector = ^T3DVector; function VectorSort( Item1 :
pointer; Item2: Pointer ) : Integer;  var    Vec1 : P3DVector;    Vec2
: P3DVector;  begin    // <0 Item1 is closer, 0=Same Distance, >0
Item2 is closer    Vec1 := Item1;    Vec2 := Item2;     result :=
(Vec2^.CamDistance - Vec1^.CamDistance);  end; procedure
TForm1.Button1Click(Sender: TObject);  var    fpo : TFPObjectList;   
idx : integer;    Item1 : T3DVector;    Item2 : T3DVector;    Item3 :
T3DVector;    Item4 : T3DVector;  begin    fpo :=
TFPObjectList.Create;    fpo.OwnsObjects := False;    
Item1.CamDistance := 111;    Item2.CamDistance := 222;   
Item3.CamDistance := 333;    Item4.CamDistance := 444;    
fpo.Add(TObject(@Item1));    fpo.Add(TObject(@Item2));   
fpo.Add(TObject(@Item3));    fpo.Add(TObject(@Item4));    
Memo1.Lines.Clear;    Memo1.Lines.Add( 'Pre-Sort' );    for idx := 0
to fpo.Count-1 do      begin        Memo1.Lines.Add(
Format('Items[%d]=%d', [idx,
P3DVector(fpo.Items[idx])^.CamDistance]));      end;    
Memo1.Lines.Add( '' );    Memo1.Lines.Add( 'Sorting the vectors' );  
fpo.Sort(@VectorSort);     Memo1.Lines.Add( '' );    Memo1.Lines.Add(
'Post-Sort' );    for idx := 0 to fpo.Count-1 do      begin       
Memo1.Lines.Add( Format('Items[%d]=%d', [idx,
P3DVector(fpo.Items[idx])^.CamDistance]));      end;     fpo.Free; 
end; 

The resulting output is:
Code: [Select]Pre-Sort
Items[0]=111
Items[1]=222
Items[2]=333
Items[3]=444

Sorting the vectors

Post-Sort
Items[0]=444
Items[1]=333
Items[2]=222
Items[3]=111


Explanation:
The TFPObjectList is storing the pointers to all of the items, in the
above case they are simply pointers to simple records existing on the
stack.

When I call the "Sort" method on the TFPObjectList object I'm
supplying the custom VectorSort comparison function that compares the
CamDistance for each of the two objects.

In a default Sort function it would "subtract" the second item from
the first item. In the above example, the VectorSort function is
subtracting the first item from the second to achieve the reverse
sort.

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bytebites · 2024-08-09 20:24

Use code tags.